This type of vehicle has the special feature of having to transport heavy loads and of being produced in the most compact possible form, because the space for vehicular traffic is limited in terms of volume. The tires used are thus as small as possible and are therefore not ideally suited to the torques that are to be transmitted.
As far as the habitual design of tires for civil engineering works machinery is concerned, the radial carcass reinforcement, anchored in each bead by being wrapped around a bead wire, is made up of at least one layer of metal reinforcing elements, the said elements being substantially parallel to one another within the layer. The carcass reinforcement is usually surmounted by a crown reinforcement made up of at least two working crown layers of metal reinforcing elements, but which are crossed from one layer to the next, making angles comprised between 15 and 70° with the circumferential direction. Between the carcass reinforcement and the working crown layers there might be layers of reinforcing elements, which are crossed from one layer to the next, and which make angles smaller than 12°, the width of these layers of reinforcing elements usually being less than those of the working layers. Radially on the outside of the working layers there are also protective layers, the reinforcing elements of which are at angles of between 10 and 65°. The crown reinforcement is itself surmounted by a tread.
What is meant by “axial” is a direction parallel to the axis of rotation of the tire, and “radial” means a direction that intersects the axis of rotation of the tire and is perpendicular thereto. The axis of rotation of the tire is the axis about which it turns in normal use.
A circumferential plane or circumferential plane of section is a plane perpendicular to the axis of rotation of the tire. The equatorial plane or circumferential median plane is the circumferential plane that passes through the center or crown of the tread and which divides the tires into two halves.
A radial plane is a plane which contains the axis of rotation of the tire.
The longitudinal direction of the tire, or circumferential direction, is the direction that corresponds to the periphery of the tire and is defined by the direction of running of the tire.
The bead wires used in the tires may be of two types. First of all, there are bead wires of the “braided” type which have a substantially circular cross section. There are also bead bundles which are formed of several layers of cords stacked on top of one another and the cross section of which may adopt various shapes.
In the case of tires for mining vehicles as described hereinabove, it is usually bead bundles that are used, these having a hexagonal cross section. Bead wires of the bead bundle type, which can be produced at a lower cost than bead wires of the “braided” type, usually have a hexagonal cross section, this being relatively close to a circle and leading to a limited bulk. These tires are usually mounted on wheels that have flat seated rims, that is to say that the surfaces of the rim against which the base of the beads of the tire are mounted have a taper of the order of 5° with respect to the axial direction.
The tires for civil engineering work machinery, like the tires described hereinabove, are usually subjected to a pressure of between 4 and 10 bar for the usual loadings and sizes.
In view of the conditions in which they are used and notably in view of the loads transported and therefore the torques which are transmitted through the tires, it has become apparent that these tires display significant wear in the region of the beads, and this limits the life of the tires. Further, while this wear is always significant on this type of tire, it can be relatively different from one tire to another, at the very least in terms of the speed with which it occurs.